1. Field of the Invention
The present invention relates to a motor for a disk drive device such as a hard disk drive. More specifically, the present invention relates to a motor having fluid lubricated dynamic bearings and gas-lubricated dynamic bearings for supporting relative rotation between a rotor and a shaft.
2. Description of the Related Part
Motors having dynamic bearings that make use of hydrodynamic pressure of a fluid lubricant are well known. Such motors include hydrodynamic bearings for supporting a sleeve member about a shaft using hydrodynamic pressure of the lubricant disposed therebetween.
One such motor is disclosed in Japanese Laid-Open patent publication Hei 8-210365. The disclosed motor includes a shaft and a sleeve member that include a pair of radial dynamic bearings and a pair of thrust dynamic bearings.
The pair of radial dynamic bearings provide support between the sleeve member and the shaft in the radial direction in a manner described below. Portions of an outer peripheral surface of the shaft radially face corresponding portions of an inner peripheral surface of the sleeve member with a small annular gap defined therebetween. The small annular gap is filled with lubricant. Dynamic pressure generation grooves are formed on either the portions of the outer peripheral surface of the shaft or on the portions of the inner peripheral surface of the sleeve member. The dynamic pressure generation grooves, the portions of the outer peripheral surface of the shaft, the portions of the inner peripheral surface of the sleeve member and the lubricant together define the pair of radial bearings. The dynamic pressure generation grooves generate dynamic pressure in the lubricant during rotation of the sleeve member relative to the shaft thereby providing stable radial support between the sleeve and shaft.
The pair of thrust dynamic bearing provide support between the sleeve member and the shaft in the axial direction in a manner described below. A thrust plate having a disk-like shaped is fixedly fitted to an portion of the shaft. Upper and lower surfaces of a thrust plate oppose adjacent surfaces of the sleeve member defining gaps therebetween. The gaps are filled with lubricant. Dynamic pressure generation grooves are formed on either the upper and lower surfaces of the thrust plate or on the adjacent surfaces of the sleeve. The dynamic pressure generation grooves, the upper and lower surfaces of the thrust plate, the adjacent surfaces of the sleeve, and the lubricant together define the pair of thrust bearings. The dynamic pressure generation grooves generate dynamic pressure in the lubricant during rotation of the sleeve member relative to the shaft thereby providing stable axial support between the sleeve and shaft.
When a motor having such dynamic pressure bearings with a lubricant rotates at a high speed, the lubricant tends to leak out of the bearings and may even leak out of the motor. As a result, there is a reduction of the dynamic pressure generated in the dynamic pressure bearings, which can make rotation of the sleeve member become unstable. In addition, the volume of the lubricant can change in response to changes in temperature. For instance, at elevated temperatures, the volume of most lubricants increases. Consequently, at elevated temperatures, lubricant leakage is more likely. Further, lubricants experience some change in viscosity in response to temperature changes. The rigidity of a bearing is likely to change as the viscosity of the lubricant changes.
In the thrust dynamic bearing, it is imperative that the shaft and the thrust plate be securely attached to one another and that the shaft be exactly perpendicular to the thrust plate. Otherwise, the rotational axis of the sleeve member may not correspond to a central axis of the shaft resulting in unwanted scraping contact between the thrust plate and the sleeve member.
In view of the above mentioned problems, motors are known that include gas-lubricated dynamic bearings. Gas-lubricated dynamic bearings utilizes air as a working fluid or lubricant. Therefore, leakage of lubricant is not a problem in a gas-lubricated dynamic bearing. Typically, a gas-lubricated dynamic bearing has a simple structure because a gas-lubricated dynamic bearing does not require a sealing structure for sealing lubricant. Also, the rigidity of a gas-lubricated dynamic bearing does not usually fluctuate in response to large changes in temperature.
However, when motors having gas-lubricated dynamic bearings are utilized in a hard disk drive device, such motors have the following problems.
For increasing memory capacity per unit area of a magnetic storage disk such as hard disk, disk drive devices have been employing a thin film magnetic head having superior magnetic characteristics or a magnetic head with an MR element (a magneto-resistive element) which responds to changes in the intensity of an adjacent magnetic field with corresponding changes in internal resistance. In this case, the MR element of the magnetic head has a thin film structure, portion of which have a generally high current density. Thus, a magnetic head with an MR element is sensitive to static electricity. However, when a disk such as a magnetic storage disk rotates in the air, electrostatic charges easily build up on the surface of the magnetic storage disk and rotary members. Therefore, when the MR element is used in a disk drive device, care must be taken to avoid discharges or grounding of electrostatic charge through the MR element of the magnetic head.
In a disk drive device having a spindle motor that utilizes gas-lubricated dynamic bearings, rotary members are supported on stationary members by means of gas pressure generated within the gas-lubricated dynamic bearings. The gas of the gas-lubricated dynamic bearing interposed between the sleeve member and the shaft member is not electrically conductive and therefore acts as an insulator. Therefore, electrostatic charges on the surface of the magnetic storage disk and the rotary members are not grounded through the rotary members to the stationary members of the spindle motor. As a result, the electrostatic charges cause problems such as a short-circuit or grounding between the surface of the magnetic storage disk and the magnetic head. Furthermore, the damping coefficient of the working fluid is not high in the gas-lubricated dynamic bearings of a spindle motor because the working fluid is the gas or air. As a result, when vibration occurs in the motor, the vibration lasts for a long period of time, which renders rotation of the spindle motor unstable.